This application claims priority to an application entitled xe2x80x9cCommon Channel Communication Device and Method Supporting Various Data Rates in a Mobile Communication Systemxe2x80x9d filed in the Korean Industrial Property Office on Sep. 14, 1998 and assigned Serial No. 98-38352, the contents of which are hereby incorporated by reference.
1. Field of the Invention
The present invention relates generally to a communication device and method for a communication system, and in particular, to a device and method for managing a common channel having various data rates in a Code Division Multiple Access (CDMA) communication system.
2. Description of the Related Art
In a CDMA communication system, a channel which is used to transmit a link establishment request from a transmission side to a receiving side is called a common channel. A common channel transmits known signals such as preamble signals prior to message transmission.
In conventional mobile communication systems, a common channel has a fixed data rate of 9.6 Kbps or 4.8 Kbps and a fixed frame size (or length) of 20 ms. Seven channels using unique orthogonal codes (e.g., Walsh codes) are used for forward common channels, and for each forward common channel, five channels separated with unique long code masks are used as reverse common channels.
With regard to operation of the common channel, the channel to be used by a base station and a specific mobile station, out of the 7 forward common channels, is determined at the base station and the mobile station using a hash function. Once the forward common channel is determined, the base station always uses the determined channel when transmitting a message to that specific mobile station. The forward common channel operates in a slotted mode or a non-slotted mode. The slotted mode is used to reduce the power consumption of a mobile station; a corresponding slot for each mobile station is determined through the hash function. When the slot is determined, a mobile station receives a message from the base station through the determined slot.
The forward common channel has a data rate fixed at either 9.6 Kbps or 4.8 Kbps, and information concerning the data rate is transmitted through a common channel message. A mobile station receiving the data rate information constantly exchanges data at the fixed data rate. In general, the forward common channel uses 80 ms slots; a message can be transmitted over two slots.
In addition, for each forward common channel, 5 reverse common channels, at maximum, can be provided. The mobile station selects the access channel at random from the corresponding channels, and a base station decodes every available reverse common channel to receive a transmitted message.
The reverse common channel has a data rate fixed at 4.8 Kbps. For reverse common channel access, the slotted Aloha method is typically used. An important factor determining the slot size is a frame size. A factor determining the slot size includes PAM_SZ and MAX_CAP_SZ, wherein PAM_SZ designates the preamble size and MAX_CAP_SZ designates the message size. The above two factors both indicate the number of frames, and are transmitted to a mobile station through an access parameter message on a forward common channel.
An access slot is comprised of a preamble (PA) of size (1+PAM_SZ) and a message capsule of size (3+MAX_CAP_SZ).
The preamble is used for sync acquisition between a base station and a mobile station. In a mobile communication system, to minimize power consumption at the mobile station and to minimize interference, unnecessary transmission is suppressed and a transmission link is only established when the mobile station has a message or data to transmit. Therefore, before arrival of the message, the base station needs to perform sync acquisition for the message to be received from the mobile station. For effective sync acquisition, prior to sending intended message or data, a mobile station transmits preambles for a predetermined time and then transmits the intended message. The preamble is a signal previously scheduled (or designated) between the base station and the mobile station. In most mobile communication systems, a mobile station can select a transmission start time of the preamble from possible transmission start times on the basis of system time information, acquired from a signal transmitted from a base station after power-on. Alternatively, the transmission start time may be determined as a fixed parameter in the system. A receiver at the base station checks for the existence of a preamble at every possible preamble transmission time, presumed on the basis of the system time. Upon detection of a preamble, the base station performs sync acquisition and tracing procedures to receive the message transmission following the preamble.
The size of a message included in the access slot is limited by the MAX_CAP_SZ parameter. The system initially sets the MAX_CAP_SZ parameter on the basis of the largest mobile station message.
The conventional method has the following problems.
First, when an access is attempted and the data rate of the common channel is fixed at 9.6 Kbps or 4.8 Kbps, the interval between access slots is also fixed so that it is not possible to reduce the delay between access attempts. Therefore, when using the conventional common channel having a fixed data rate, mobile stations may conflict due to the constant slot interval. Furthermore, an increase in access delay may cause difficulties in data service, in the light of state transitions occurring during the data service.
Second, in the case where, during data service, the system transitions to a state where the dedicated channel has terminated, and there is a small amount of data to be transmitted at once, the resources consumed in the additional process of restarting data transmission are greater than the resources required for actual data transmission, thereby causing an ineffective use of the resources. That is, sometimes it is required to transmit data frames smaller than a predetermined size without reassignment of a dedicated channel, and it is difficult to transmit data frames of various sizes using the common channel with a fixed data rate.
It is, therefore, an object of the present invention to provide a device and method for realizing an access adaptively by varying the interval between access slots according to the data rate in a CDMA communication system.
It is another object of the present invention to provide a device and method for transmitting a data frame smaller than a predetermined size through a common channel according to the amount of data to be transmitted in a CDMA communication system.
It is further another object of the present invention to provide a device and method for reducing the limitation on the rate and size of data which can be transmitted over a common channel when a reverse access slot is reduced in size, to ensure effective use of resources and fast access in a CDMA communication system.
To achieve the above and other objects, there is provided a common channel communication method for a base station in a mobile communication system including common channels each supporting at least two data rates. The method comprises determining a data rate of a common channel available in a common channel service state and a frame length serviceable at the determined data rate, including information about the determined data rate and frame length on a specific forward common channel message before transmission; and upon receipt of an acknowledge message through a specific reverse common channel, setting a data rate and a frame length of the common channel to the determined values.